LAGOS, Nigeria – At a crowded public hospital, Seun Dupe’s twin boys squirm underneath a bundled-up mosquito net as they share the same bassinet. Already they are part of the swelling tide of history, as the U.N. on Monday marked the world population reaching 7 billion amid fears of how the planet will cope.

That strain is already apparent at Lagos Island Maternity Hospital, where the droning roar of a generator could be heard throughout one hot ward as it powered the slowly rotating ceiling fans and incubators. While Nigeria is oil-rich, its power grid doesn’t produce anywhere near enough electricity for its more than 160 million people.

Dupe, a 32-year-old hairdresser, remains an optimist even as experts warn of the staggering burden facing Africa’s most populous nation and other developing countries. Officials say Nigeria’s megacity of Lagos is expected to surpass Cairo as the continent’s most populous.

“Where there is life, there is hope,” says Dupe, who has yet to decide on her boys’ names. “So I know Nigeria will be a great nation.”

Amid the millions of births and deaths around the world each day, it is impossible to pinpoint the arrival of the globe’s 7 billionth occupant. But the U.N. chose Monday to mark the day with a string of festivities worldwide, and a series of symbolic 7 billionth babies being born.

In South Africa, Nozipho Goqo, an unemployed 19-year-old from Johannesburg, gave birth to a boy Monday. She gave him a Zulu name – Gwakwanele – that means “enough.”

A nurse at Charlotte Maxeke, a sprawling teaching hospital, teased Goqo that she was too young to know whether Gwakwanele would be her last. Goqo smiled, and said she was sure.

Across the maternity ward, Dora Monnagaratoe cuddled her newborn son in a bed. The 40-year-old maid named her fourth baby Tebogo, or “we are thankful” in the Sotho language.

Demographers say it took until 1804 for the world to reach its first billion people, and a century more until it hit 2 billion in 1927. The twentieth century, though, saw things begin to cascade: 3 billion in 1959; 4 billion in 1974; 5 billion in 1987; 6 billion in 1998.

The U.N. estimates the world’s population will reach 8 billion by 2025 and 10 billion by 2083. But the numbers could vary widely, depending on everything from life expectancy to access to birth control to infant mortality rates.

In Uttar Pradesh, India – the most populous state in the world’s second-most populous country – officials said they would appoint seven girls born Monday to symbolize the 7 billion.

India, which struggles with a deeply held preference for sons and a skewed sex ratio because of millions of aborted female fetuses, is using the day to highlight that issue.

“It would be a fitting moment if the 7 billionth baby is a girl born in rural India,” said Dr. Madhu Gupta, an Uttar Pradesh gynecologist. “It would help in bringing the global focus back on girls, who are subject to inequality and bias.”

According to U.S. government estimates, India has 893 girls for every 1,000 boys at birth, compared with 955 girls per 1,000 boys in the United States.

On Monday, the chosen Indian babies were being born at the government-run Community Health Center in the town of Mall, on the outskirts of the Uttar Pradesh capital of Lucknow.

Six babies were born from midnight to 8 a.m. Monday. Four were boys.

China, meanwhile, which at 1.34 billion people is the world’s most populous nation, said it would stand by its one-child policy, a set of restrictions launched three decades ago limiting most urban families to one child and most rural families to two.

“Overpopulation remains one of the major challenges to social and economic development,” Li Bin, director of the State Population and Family Planning Commission, told the official Xinhua News Agency. He said the population of China would hit 1.45 billion in 2020.

While the Beijing government says its strict family planning policy has helped propel the country’s rapidly growing economy, it has also brought many problems. Soon, demographers say, there won’t be enough young Chinese to support its enormous elderly population. China, like India, also has a highly skewed sex ratio, with aid groups saying sex-selective abortions have resulted in an estimated 43 million fewer girls than there should be, given the overall population.

India, with 1.2 billion people, is expected to overtake China around 2030 when the Indian population reaches an estimated 1.6 billion.

In Nigeria, large families remain the norm in rural areas of its Christian south and Muslim north, though some are beginning to have smaller families. Most who enter the Lagos Island Maternity Hospital plan for around three or four children, said Dr. Femi Omololu, its acting managing director. The hospital offers family planning advice, though husbands often don’t attend those discussions, he said.

“Nigerians like large family sizes, but with the economy, it’s tough to have,” Omololu said.

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Associated Press writers Donna Bryson in Johannesburg; Tim Sullivan in New Delhi; and Biswajeet Banerjee in Lucknow, India, contributed to this report.